Nullarbor Plain was wet and forested 3 to 5 million years ago, research reveals

The Nullarbor was once cloaked in gums and banksias and enjoyed a rainfall much higher than today.

Not for nothing does Nullarbor Plain mean "no trees": the vast expanse, spanning 200,000 square kilometres along the Great Australian Bight, with the Great Victoria Desert to its north, supports little more than sparse saltbush shrubs and clumps of hardy grasses.

Yet today's flat and arid plain was once blanketed with forests of gums, banksias and other flowering plants, a team of Melbourne scientists has discovered.

Using new techniques for deciphering Australia's environmental history, the researchers reveal that between 3 and 5 million years ago the Nullarbor Plain was at least three times wetter than it is today – and probably home to a range of small animals such as possums.

The ground-breaking research refutes the widely held view that the area's dry climate developed progressively over the past 14 million years, in response to the cooling of Antarctica.

Flat and devoid of trees, the Nullarbor Plain is a very different place today from its wet-gum-and-banksia past, more than 3 million years ago.

Photo: Adam Bruzzone

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"It has been very difficult to document the nature of that drying because the arid climate has tended to destroy organic remains that might have become fossils," said Melbourne University palaeoclimatologist Dr Kale Sniderman, who led the research.

"Additionally, even where we have fossils, it has been hard to accurately determine how old they are."

As yet, the scientists have direct evidence only for plants that were present. "But, since there's evidence for large-scale expansion of forests into what are now arid lands, characteristic forest-dependent animals must also have expanded their geographic ranges and population sizes," Dr Sniderman said.

Researchers abseil down the vertical entrance of one of the hundreds of caves on the Nullarbor Plain.

Photo: Jon Woodhead

The Nullarbor Plain is a former shallow seabed, as indicated by a range of calcareous skeletons that make up the Earth's largest continuous slab of limestone. "The limestone was laid down beneath the sea at various times between about 50 and 20 million years ago," Dr Sniderman explained.

Crustal movements lifted the area about 14 million years ago, after which wind and rain probably smoothed much of it out.

One of the team members, Professor Jon Woodhead, has spent a decade perfecting methods to date these formations, most of them more than 500,000 years old.

Professor Woodhead found that most speleothems there had grown between 6 and 3 million years ago. "This implied that the region was much wetter than today because speleothem growth depends on water dripping through a cave – and today such growth does not occur in caves beneath the arid Nullarbor surface," Dr Sniderman said.

To clinch the deal, Dr Sniderman extracted pollen grains that were preserved, in very small concentrations, within some of the Nullarbor speleothems.

Stalagmite formations, created between 3 and 6 million years ago, when the Nullarbor had a much wetter climate than today.

Photo: Jon Woodhead

"By combining these two approaches, we could infer the nature of the vegetation in the Nullarbor region between about 5 and 3 million years ago," he explained. "And, by implication, we deduced the nature of the much wetter climate supporting that vegetation."

Today, the Nullarbor Plain receives an average of 250 millimetres of rain each year. But before 5 million years ago the rainfall was approximately 480 millimetres, he said.

A dense ceiling of stalactites within an ancient cave on the Nullarbor Plain.

Photo: Jon Woodhead

During the "big wet", between 5 and 3.5 million years ago, precipitation rose to an estimated 1220 millimetres.

"In the space of just 100,000 years, the whole area became a forest of gums and banksias – which suggests a rainfall of at least two or three times higher than today," Dr Sniderman said.

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